The invention relates to an electrolytic cell for the production of aluminum by the fused salt electrolytic process where the said cell comprises a pot containing liquid, precipitated metal and above this a molten electrolyte into which at least one anode dips from above.
To produce aluminum by the electrolytic reduction of aluminum oxide, the latter is dissolved in a fluoride melt which is made up for the greater part of cryolite. The cathodically precipitated aluminum collects under the fluoride melt on the carbon floor of the cell, the surface of the liquid aluminum itself forming the cathode. Anodes which in conventional reduction processes are made of amorphous carbon are secured to an overhead anode beam and dip into the melt from above. At the carbon anodes oxygen is produced as a result of the electrolytic decomposition of the aluminum oxide. This oxygen combines with the carbon in the anodes to form CO and CO.sub.2. Electrolysis takes place generally in a temperature range of about 940.degree.-970.degree. C., whereby the electrolyte becomes depleted in aluminum oxide. At a lower concentration of about 1-2 wt.% aluminum oxide in the electrolyte the anode effect occurs, causing an increase in voltage from e.g. 4-4.5 V to 30 V and more. At this time at the latest the crust of solidified electrolyte must be broken open and the concentration of aluminum oxide increased by the addition of fresh aluminum oxide (alumina).
Under normal operating conditions the cell is usually serviced at regular intervals, even if no anode effect occurs, and involves breaking open the crust and adding alumina.
It is known that when large currents are drawn that is higher than 50 kA (kiloampere), the combined effect of vertical components of the magnetic field and horizontal components of the electric current can cause undesirable deformation of the surface of the metal on the floor of the cell, and can lead to undesirable strong flowing of the metal. When the interpolar distance is small this deformation can be so pronounced that aluminum touches the anodes and causes short-circuiting. The flowing of the metal at the surface can also lead to increased chemical dissolution of the aluminum in the electrolyte which, as is well known, results in less efficient use of the applied current. It is therefore impossible to operate with interpolar distances smaller than a certain critical limit. On the other hand the loss of electrical energy is greater the larger the interpolar distance at the same current density. In principle a reduction in current density would be advantageous, however, this would, require unacceptably high investment costs for the cells and the pot room.
Besides various measures and constructions for reducing the vertical components of the magnetic field and the horizontal components of the electrical current, cathode constructions are known whereby these are wet by aluminum and feature a thin layer of aluminum which moves only little in the direction perpendicular to the cathode arrangement, as a result of which the classical surface deformations, both stationary doming and moving waves, are to a large extent eliminated. These wettable materials are, however, very expensive and still have to be shown to have a long service life.
The greatest disadvantage of these arrangements is, however, that the circulation of the electrolyte between anode and cathode is made more difficult and causes the cryolite melt to become depleted in alumina as the aluminum is separated out, thus making the cell prone to the anode effect.
According to U.S. Pat. No. 4,071,420 the circulation of the cryolite melt is improved by the cathode elements, which are in the form of pipes closed at the bottom, projecting, in the region of the anodes, out of the liquid aluminum collected on the rest of the floor of the cell. The pipes are filled completely with aluminum and the interpolar distance can be kept small. The additional amounts of metal produced by the electrolytic process flow into a lower lying sump of liquid aluminum.
There has to be electrical contact between the carbon floor and the aluminum in the above mentioned pipes which are closed at the bottom. The contact may be achieved by the pipe being made of an electrically conductive material or by the aluminum being in direct contact with the conductive cell floor. Apart from the difficult and therefore expensive production of the wettable pipes, this arrangement is effective only when the surface area of aluminum facing the anode is small. This means that the ratio of wettable material to area serving as cathode is high. There is, therefore, no substantial cost savings over other, known cathodes made of wettable materials.
In U.S. patent application Ser. No. 209,124, a cathode arrangement for a molten salt electrolytic cell with relatively small interpolar distance is described, in which there is a granular particulate material in the liquid metal and apart from a top-most, freely moveable layer of at least 2 mm movement of the metal is effectively hindered. The interpolar distance, however, is large enough to ensure free circulation of the molten electrolyte.
It is a principal object of the present invention to further improve the molten salt electrolytic cell described in the foregoing U.S. patent application, in particular with respect to its effects on the environment, length of useful service and economics.